Update: Saturday 6:35am

When you are in the midwest, you never like to hear about severe hail and tornadoes... but we are. Once we get through Sunday, the weather will improve considerably. It's late afternoon and evening Sunday and into the early morning hours of Monday (midnight to 3-4am) that is the concern. If you have enclosed trailers for your car, I would certainly use them on Sunday.

EARLY IN THE PERIOD...ELEVATED THUNDERSTORMS MAY BE ONGOING ALONG THE NOSE OF LLJ (Low level Jet) ACROSS THE UPPER RED RIVER VALLEY. THIS ACTIVITY SHOULD BE ROOTED AOA (at or above) 850MB (~5000 feet) AND THE GREATEST THREAT WITH THESE STORMS SHOULD BE MARGINALLY SEVERE HAIL BEFORE THEY PROPAGATE INTO SRN MANITOBA.

HOWEVER... GREATER SEVERE RISK WILL OCCUR ALONG THE COLD FRONT DURING THE LATE AFTERNOON AS BOUNDARY LAYER WARMS... FORCING SBCAPE IN EXCESS OF 3000 J/KG FROM CNTRL MN...SWWD INTO KANSAS. LATEST THINKING IS FRONTAL CONVECTION SHOULD DEVELOP AFTER 21Z (5pm) WITHIN AN INCREASINGLY SHEARED ENVIRONMENT THAT SHOULD INITIALLY SUPPORT SUPERCELL STRUCTURES. ACTIVITY ACROSS CNTRL KS MAY BE SOMEWHAT HIGH-BASED AS TEMPERATURES SOAR INTO THE 90S. HOWEVER...UPDRAFTS THAT EVOLVE ACROSS NEBRASKA INTO MN WILL BE CONSIDERABLY LOWER AND MULTIPLE HP SUPERCELLS COULD DEVELOP ALONG THE FRONT BEFORE STORM MERGERS RESULT IN AN ELONGATED MCS-TYPE COMPLEX.

LARGE HAIL IS LIKELY WITH THIS ACTIVITY AND DAMAGING WINDS MAY ALSO BE NOTED. A FEW TORNADOES CAN NOT BE RULED OUT WITH EARLY DISCRETE SUPERCELL STRUCTURES. FRONTAL MCS SHOULD PROGRESS INTO THE MID-UPPER MS VALLEY DURING THE OVERNIGHT PERIOD.

Click for full size - Saturday Morning Water Vapor Image. Note the drier air (darker color) that will be moving into Lincoln today. Also note the blob over Montana, that is Sunday evenings weather maker.

THE MAIN SHOW WILL PROBABLY WAIT UNTIL LATE AFTERNOON OR EARLY EVENING AND BE MORE TIED TO LOW LEVEL CONVERGENCE ALONG THE APPROACHING COLD FRONT. AGREE WITH SPC DAY 2 OTLK WHICH HAS ALL OF OUR AREA IN A SLGT RISK.

ML CAPE VALUES ARE EXPECTED TO REACH 3000 TO 4000 J/KG IN THE AFTN AND LINGER INTO THE EVENING. 0 TO 6 KM BULK SHEAR VECTORS IN COMBINATION WITH EXPECTED CAPE SHOULD BE SUFFICIENT FOR STORM ORGANIZATION. INITIALLY DISCRETE STORMS WILL LIKELY MERGE INTO ONE OR MORE CLUSTERS AND/OR A LINE. 21Z SUNDAY TO 06Z MONDAY (5pm until 2am ) SEEMS TO BE THE MOST FAVORED TIME BASED ON FORECAST SOUNDINGS AND OTHER CURRENT MODEL DATA. IN ADDITION TO THE SEVERE THREAT...LOCALLY HEAVY RAIN IS PROBABLY GOING TO BE AN ISSUE DUE TO THE RECENT ISSUES WITH SOME STREAMS/RIVERS.

The first two images show the bulk shear and MLCAPE parameters that they refer to in their discussion. The last image is a Supercell Composite parameter which factors in a combination of other severe weather parameters. Looks pretty ugly over Lincoln.

0-6 km Shear - Shear is the change in wind direction with height.

Convective Available Potential Energy. A measure of the amount of energy available for convection. CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg. Mean Layer CAPE - CAPE calculated using a parcel consisting of Mean Layer values of temperature and moisture from the lowest 100 mb above ground level.

SBCAPE (Surface-Based Convective Available Potential Energy) is a measure of instability in the troposphere. This value represents the total amount of potential energy available to a parcel of air originating at the surface and being lifted to its level of free convection (LFC). No parcel entrainment is considered. The CAPE and CIN calculations use the virtual temperature correction.

A multiple ingredient, composite index that includes effective storm-relative helicity (ESRH, based on Bunkers right supercell motion), most unstable parcel CAPE (muCAPE), and effective bulk wind difference (EBWD). Each ingredient is normalized to supercell "threshold" values, and larger values of SCP denote greater "overlap" in the three supercell ingredients. Only positive values of SCP are displayed, which correspond to environments favoring right-moving (cyclonic) supercells.

ALTHOUGH ELEVATED CONVECTION COULD PERSIST INTO SUN AFTN AIDED BY LOW LEVEL JET...PER NAM/GFS...12Z ECMWF GENERALLY KEPT AREA DRY UNTIL 00Z MONDAY WHEN STRONG UPPER JET PUNCHES INTO REGION ON SOUTH SIDE OF THIS NEXT UPPER TROUGH. DUE TO UNCERTAINTY IN AMOUNT OF ELEVATED CONVECTION PRECEDING MAIN LATE AFTN/EVENING DEVELOPMENT LEFT IN MORNING POPS NW 2/3RDS AND DID NOT BREAK UP THE AFTN PERIOD TO ACCOUNT FOR POSSIBLE HIGHER CHANCES LATE IN THE AFTN. DID BOOST MAX TEMPS A FEW DEGREES SOUTH ON SUNDAY AS THAT AREA DID SEEM THE LESS SUSCEPTIBLE TO IMPACT OF MORNING WARM ADVECTION CLOUDS PSBL PRECIP. STRONG/SEVERE STORM CHANCES APPEAR HIGHER THAN PAST SEVERAL DAYS AS 0-6KM SHEAR IS STRONGER AND MAX TEMPS NEARING 90 SHOULD INCREASE MLCAPE VALUES. ALTHOUGH HEAVY RAIN WILL LIKELY OCCUR WITH THE ACTIVITY AS WELL...VERY HARD TO PIN DOWN WHERE AXIS OF HEAVIEST WILL OCCUR AS A CASE COULD BE MADE FOR IT SETTING UP JUST ABOUT ANYWHERE IN OR EVEN POSSIBLY JUST OUTSIDE THE FA.

Previous Post

Beginning to become a little more concerned about instability during the day and afternoon on Sunday as a shortwave rotates through the base of the trough. It now appears that there will be the possibility for some thunderstorms to popup. The majority of the bad weather should remain to the east and north, but the severe parameters are worth noting. Here is an except from this mornings AFD from Omaha NWS. I bolded the important part.

.LONG TERM...(SATURDAY NIGHT THROUGH WEDNESDAY) ISSUED AT 327 AM CDT THU AUG 28 2014 THERE IS A BREAK IN THE STORMY WEATHER SATURDAY NIGHT... HOWEVER THUNDERSTORM CHANCES INCREASE SUNDAY AFTERNOON AND SUNDAY NIGHT. DO MAINTAIN LIKELY POPS DURING THIS TIME WITH A SHORTWAVE TROF PUSHING THROUGH THE PLAINS AND HIGH PWATS RETURN AHEAD OF THE FRONT. HAVE A DRIER PERIOD FORECAST MONDAY THROUGH WEDNESDAY.

BOTH MODELS DEPICT THE ADVANCE OF A FAIRLY STRONG UPPER TROUGH INTO THE CENTRAL U.S. DAY 4 /SUN 8-31/...ALONG WITH AN ASSOCIATED/SHARP COLD FRONTAL PROGRESSION.

THE MOST SUBSTANTIAL SEVERE RISK THIS PERIOD REMAINS APPARENT DAY 4 FROM THE CENTRAL PLAINS NEWD TO THE UPPER MS VALLEY...AS THE UPPER TROUGH SHIFTS INTO/ACROSS THIS AREA. AS THE ASSOCIATED COLD FRONT ADVANCES INTO A MOIST/DESTABILIZING AFTERNOON AIRMASS...VIGOROUS THUNDERSTORM DEVELOPMENT IS EXPECTED TO OCCUR. WITH A BELT OF 40 TO 50 KT SWLY FLOW PROGGED TO SPREAD ACROSS THE MID MO VALLEY VICINITY THROUGH THE DAY ATOP LOW-LEVEL WARM-SECTOR SSELYS...SHEAR WILL BECOME FAVORABLE FOR ROTATING STORMS. AS A RESULT...A RISK FOR LARGE HAIL AND LOCALLY DAMAGING WINDS -- AND POSSIBLY A FEW TORNADOES -- SEEMS LIKELY TO MATERIALIZE...PARTICULARLY WITHIN A ZONE FROM NRN KS NNEWD INTO MN/WRN WI DURING THE AFTERNOON AND EARLY EVENING HOURS...AND THEN INTO IA/NWRN MO THROUGH THE EVENING HOURS.

This is a 6 hour accumulated precipitation map above. What that means is this is what the GFS expects to happen from 2pm until 8pm in the evening. Obviously the bulls-eye for rain isn't too far away so this is something to be watched.

This is vertical velocity and vorticity, the lift and twist to the air as it rises. These are from two different levels. The left is an 850mb view (~5000 feet) and the right is a 500mb view (~18,000 feet). Note that they only line up to the north of the state, meaning the most likely area for severe is to the north.

The parameters exist for some High Precipitation (HP) supercells, this is 11am Sunday. Again, something we'll be watching for as we get closer to Sunday.

Updated 9:17 pm, Thursday

In case my regular readers are wondering what in the heck I'm talking about...The SCCA (Sports Car Club of America) Solo National Championship is held in Lincoln Nebraska starting this weekend and running through next weekend. It is billed as the largest motor sports event in the world with almost 1200 competitors this year. People come from around the country and Canada to try to win the coveted "Jacket" and a National Championship. I've talked about this sport in previous blog post, but here's a link just in case you missed it. Oh... and I'm participating. :-)

Weather is always an important factor in any type of racing, and this one is no different. So with that, we going to take a preliminary look at what may happen. Things will change and I'll be making updates as necessary.

We'll start by taking a look at the Area Forecast Discussion issued by the Omaha NWS office:

.LONG TERM...(FRIDAY NIGHT THROUGH TUESDAY) ISSUED AT 318 AM CDT WED AUG 27 2014 A FEW SHOWERS LINGER INTO FRIDAY NIGHT...BUT SATURDAY LOOKS MAINLY DRY. THERE IS STILL SOME POCKETS OF MOISTURE AND ELEVATED INSTABILITY SO WILL NEED TO KEEP AN EYE ON THIS. LIFT INCREASES SATURDAY NIGHT THROUGH SUNDAY NIGHT AS ANOTHER SHORTWAVE TROF AFFECTS THE AREA WITH A GOOD CHANCE FOR RAIN SUNDAY NIGHT.

What this means is that the weak ridge should bring decent weather for the Pro Solo Championship, but some showers may develop Sunday and into Monday as a trough enters the state and begins to flatten out or weaken. Shortwaves rounding the base of the trough will trigger some light showers through the period... but the timing is unknown right now. There doesn't appear to be any major rain events for the beginning of the event, all of that appears to stay to the north and east. But images speak louder than words (my words anyway...) to here is a visual look at what may happen.

I'm going to save the Tue/Wed forecast for another day or so in order to get a clearer picture of the way things will evolve.

These are the temperatures for Saturday around 2pm. If you like low 80's you'll like Saturday. It may start off cloudy with a few showers around, but start to clear as the weak low in the upper right of the image, moves to the northeast.

Sunday brings a mixed bag as the trough swings southeast from South Dakota. Notice the temperatures range from 90 to the mid 70's.

6 hour accumulated precipitation ending at 2pm Sunday. It may be damp in the morning hours as a shortwave passes through.

Again Monday at 8am. This would indicate showers occurred overnight. Monday appears to be a mostly cloudy day at this point.

Temps on Monday. Luckily we're on the cooler side of things. 90's to the south. Note that Lincoln is just north of the front with winds out of the northeast. Also note the 1004mb contour, as that piece of energy may bring some showers to the area.

Some of you have no idea what the SCCA Atlanta Match Tour is... so a little explanation is in order. First, it's an autocross and I've made several blog post about what that is over on my autocross blog, but the Match National Tours are run with a slightly different format than a normal Tour. For more information about that, click on this link.

Anyway... THIS blog post is about the weather for that event which is being held next weekend, May 17-18th down at Atlanta's Turner Field Green Lot, the one with the big blue wall. If you'd like to see some of the best drivers from around the country, stop by and visit. You'll need to sign a waiver but it's free to watch. Either way, here is next weekends outlook as it is right now.

Weather in the southeast can be difficult to forecast at times during the spring and summer when moisture and pop-up thundershowers can show up just about anywhere. Next weekend is going to have at least two pretty good days, and maybe all three. But this morning I noticed a little feature that could possibly have an impact on the weather at some point during the three day weekend. Hopefully if there is any impact from this in the form of precipitation, it will be during the evening and overnight hours and a non-factor for all of the competitors. It is the fly in the ointment for next weekend and something to keep an eye on. But overall, I'm not so sure that at this point, we could have ordered a better weekend for this event.

Friday

GFS Temps Friday 2AM - Click to enlarge

After a low chance rain week, temperatures will cool down sharply behind a fairly strong cold front, especially for this time of year. The image to the left shows temperatures on Friday at 2AM... and 40's are pretty cold for this time of year. High pressure sitting to our west will bring a NW flow to the metro area during the day on Friday, keeping afternoon temperatures in the low to mid 70's along with low humidity readings. It will be a great day to get in some practice runs and get use to the Turner Field surface. I'll see you Saturday... :-)

GFS Temps Friday 2PM - Click to enlarge

Saturday/Sunday

GFS Temps 8AM Sat/Sun - Click to enlarge

It appears that the front may stall just south of the metro area overnight on Friday into Saturday. Temperatures and moisture increase Saturday as we get in a westerly flow. The image to the left shows 8AM temperatures on Saturday/Sunday mornings. You notice that the Saturday morning temperatures are warmer than both Friday and Sunday. Temps Saturday afternoon will rebound back to the upper 70's, but drop back down on Sunday to the lower 70's.

Precipitable water values increase, with a larger area of 1" values pass over central Georgia during the day on Saturday and creep toward the metro area as the day goes on.

GFS Temps 2PM Sat/Sun - Click to enlarge

The only thing that I worry about, is that there is a blob of higher PW values passing the metro area Saturday evening, This is in conjunction with an upper level trough that will be swinging through to our north. Most of the energy from the trough stays well north of us, but could still trigger a shower or two as it passes by (see the image at the very beginning of this post). Timing would be everything, and as it looks right now, the evening hours would be the most likely time for this to happen, at least for now. Things can and will change as we get closer, so we'll keep an eye on this feature.

GFS 500MB/Precipitable Water Sat 8PM - Click to enlarge

Sunday should be a great day with highs in the low 70's and much lower dew point values. A great day to autocross!

GFS Sunday Temps and Humidity - Click to enlarge

We are still at the limits of what the models are any good at, so many of the details will change between now and Friday. I'll take a look in another day or two and see how it stands then.